The present invention generally covers receivers in wireless communications systems, and more specifically is generally drawn to addressing noise and/or interference effects exhibited by received signals, where the signals were transmitted via a transmitter employing high power amplifiers (HPAs), such as satellite transponders in a satellite communications system. A satellite communication system may include a transmitter having a high power amplifier (HPA) or a transponder that includes a transmitter having an HPA. The output of a transmitter can be seen as a sequence of symbols called a phrase. Each symbol represents a sequence of bits (e.g., in the case of 8PSK, each symbol represents 3 bits), and the transmitter will output the phrase one symbol at a time during transmission. As a transmitter shifts from one symbol to the next in the phrase, previous output symbols may cause interference in the output of the current symbol. Similarly, the current symbol is also affected by interference resulting from subsequent or future symbols. This interference in the current symbol caused by previous as well as symbols is referred to as the inter-symbol interference (ISI). ISI represents a form of signal distortion whereby one symbol interferes with subsequent symbols. ISI is usually caused by multipath propagation, or the inherent non-linear frequency response of a channel causing successive symbols to blur together. Further, typically, an HPA operates most efficiently at or near saturation, however, operation of an HPA at or near saturation generates nonlinear distortion in output channels. ISI can be mitigated by reducing the transmission or throughput rate of the transmitter, however, a reduction in the throughput rate proportionately reduces bandwidth efficiency.
In order to increase system throughput, a logical goal would be to maximize the number of transponders/HPAs of the satellite transmission antenna. Due to physical limitations, however, there is a maximum number of HPA units that can fit in a single transponder. To combat this issue, multiple carriers can be shared by a single transponder HPA (multicarrier operation), allowing for the transmission of more data and the servicing more users without exceeding the physical limitation on the number of HPAs per transponder. Another benefit of multicarrier operation is that it facilitates a reduction of the transmission symbol rate per carrier without sacrificing system throughput, which greatly eases the burden on hardware implementation. In a multicarrier system, however, the amplification of multiple carriers by way of a single HPA (driven at or near its saturation point for maximum efficiency) generates a large amount of nonlinear interference or distortion, which further contributes to performance degradation issues.
Additionally, in order to increase transmission throughput, the transmission rate or symbol rate (in the time domain) can be increased, without altering the spectral shape of the signal. Increasing the transmission throughput, however, further exacerbates ISI issues. According to the Nyquist theorem, there is an ideal transmission limit (the Nyquist rate) beyond which the ambiguity in ability to resolve symbols at the receiver increases—the maximum number of code elements per second that could be unambiguously resolved at the receiver. Transmission at the Nyquist rate mitigates ISI, while increasing the transmission throughput above the Nyquist rate (at a “faster than Nyquist (FTN)” rate) resulting in linear interference that exacerbates the issues of ISI.
Further, in order to increase spectral efficiency, it is desirable to pack channels closer together in the frequency domain, which results in increased throughput (e.g., in bits/second/Hz, where the Hz reflects the distance between adjacent channels). The spectral efficiency, however, is constrained by the roll-off factor, which reflects the rate of slope or steepness of a transmission function with respect to frequency. The slower the roll-off rate (or the higher the roll-off percentage or factor) the further apart the adjacent channels must be placed to mitigate adjacent channel interference (ACI). ACI results from extraneous power picked up from a signal in an adjacent channel (e.g., one channel bleeds-over into an adjacent channel). Accordingly, the slower the roll-off rate of a channel, the higher the signal power that can be picked up by an adjacent channel. Therefore, there is an inherent tradeoff between roll-off rate and spectral efficiency.
Accordingly, to maximize bandwidth efficiency of a system, two goals are to increase transmission throughput of a transponder (transmission rate) in the time domain, and to increase the rate or steepness of the roll-off (operate at a decreased or minimized roll-off factor or percentage). As described above, however, an increase in the transmission throughput beyond certain levels and tightening the roll-off contributes to both ISI and ACI. More specifically, the resulting interference manifests itself as a structured interference, which is significant and extends for a relatively longer period in the time domain (the interference tends to linger in time over many symbols, resulting in a significant degradation in performance). At the receiver, in view of the lengthened period of significant interference, the receiver must be configured to handle the increased interference levels, which would require increased complexity in the receiver. The longer the interference memory, the receiver must account for the possible sequences, which is exponential in the symbol alphabet over that memory. For example, with a 16APSK modulation scheme, the receiver would be required to consider 16 raised to the power of the channel interference memory signal possibilities in the decoding process. In other words, the receiver must be configured to account for a significantly increased number of possibilities for the transmitted signal before making a decoder decision.
Further, due to physical limitations of the satellite, there are a maximum number of HPA units that can fit in a transponder. To solve the issue of such physical limitations, sharing multiple carriers by a single transponder HPA (multicarrier operation) allows for transmitting more data and servicing more users. Another benefit of multicarrier operation is that it allows for reducing the transmission symbol rate per carrier without sacrificing system throughput. This greatly eases the burden on hardware implementation. When multiple carriers are amplified by way of a single HPA, and when the HPA is driven near its saturation point, a significant level of nonlinear interference is generated. Interference is an undesirable result of increasingly crowded spectrum, when multiple carriers share the same transponder high power amplifier (HPA). The transponder HPA transmits a maximum signal strength when operating at or near its saturation output power level. Operating near saturation, however, increases nonlinearities in the HPA, and such nonlinearities in the HPA result in nonlinear distortion (e.g., intermodulation distortion (IMD), which comprises unwanted amplitude and phase modulation of signals containing two or more different frequencies in a system with nonlinearities). The intermodulation between each frequency component will form additional signals at frequencies that are not, in general, at harmonic frequencies (integer multiples) of either, but instead often at sum and difference frequencies of the original frequencies. The spurious signals, which are generated due to the nonlinearity of a system, are mathematically related to the original input signals. When the spurious signals are of sufficient amplitude, they can cause interference within the original system or in other systems, and, in extreme cases, loss of transmitted information, such as voice, data or video.
IMD causes interference within a message itself as well as between the message signals by transferring modulations from one frequency range to another. The problem is particularly acute when a cost effective nonlinearized HPA is operated with minimal output back-off (OBO). OBO is the amount (in dB) by which the output power level of the HPA is reduced, or “backed-off,” from the saturation output power level. The problem is further compounded when the carriers passing through the HPA are bandwidth efficient, whose constellations include multiple concentric rings, and the carriers are tightly spaced within the limited spectrum. The interference issues are further complicated when transmission throughput of a transponder (the symbol transmission rate) is increased in the time domain (e.g., an FTN rate) and the rate or steepness of the roll-off is increased. As described above, however, an increase in the transmission throughput beyond certain levels (e.g., the Nyquist level) and tightening the roll-off contributes to both ISI and ACI.
Band-pass filtering can be an effective way to eliminate most of the undesired products without affecting in-band performance. However, third order intermodulation products are usually too close to the fundamental signals and cannot be easily filtered. The amplitude and phase distortion is unacceptable in systems that use higher order modulation schemes, because the distortion results in an error component in the received vector, degrading the receiver's bit error rate (BER). Other attempts to compensate for nonlinear interference have been complex and require receivers to exchange information. For instance, a conventional system compensates for linear and nonlinear ISI) and linear and nonlinear adjacent channel interference (ACI) due to the nonlinearlity of HPA and tight crowding of carriers in a transmitter HPA or transmitter section of a transponder HPA. However, such a system requires receivers to coordinate samples from adjacent carriers, resulting in increased system complexity and computational effort.
What is needed, therefore, is an approach for increasing the transmission throughput of a wireless transmitter or transponder HPA driven at or near saturation, while being able to efficiently decode the transmitted signal at a receiver.